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Postfix Documentation
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Postfix Built-in Content Inspection

Built-in content inspection introduction

Postfix supports a built-in filter mechanism that examines message header and message body content, one line at a time, before it is stored in the Postfix queue. The filter is usually implemented with POSIX or PCRE regular expressions, as described in the header_checks(5) manual page.

The original purpose of the built-in filter is to stop an outbreak of specific email worms or viruses, and it does this job well. The filter has also helped to block bounced junk email, bounced email from worms or viruses, and notifications from virus detection systems. Information about this secondary application is given in the BACKSCATTER_README document.

Because the built-in filter is optimized for stopping specific worms and virus outbreaks, it has limitations that make it NOT suitable for general junk email and virus detection. For that, you should use one of the external content inspection methods that are described in the FILTER_README and SMTPD_PROXY_README documents.

The following diagram gives an over-all picture of how Postfix built-in content inspection works:

Network or
local users
-> Built-in
-> Postfix
-> Delivery
-> Network or
local mailbox
Undeliverable mail
Forwarded mail

The picture makes clear that the filter works while Postfix is receiving new mail. This means that Postfix can reject mail from the network without having to return undeliverable mail to the originator address (which is often spoofed anyway). However, this ability comes at a price: if mail inspection takes too much time, then the remote client will time out, and the client may send the same message repeatedly.

Topics covered by this document:

Postfix Documentation
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